Licensed Computer Geek
This is a funny little minor bug that doesn't really harm anything. Try this: grab one of your dock icons (any one will do.) Drag it slowly to just a few pixels above the Dock's upper border (or side, or bottom, depending on how you've got it positioned.) If done correctly, the Dock will sort of pulsate as the cursor repeatedly oscillates between being inside and outside the Dock. You'll have to try it to see what I mean. This only works if you have your Dock set to full size, so it grows and shrinks based on the number of items in it.